1. Field of the Invention
The present invention is related to methods and apparatuses for knee replacement surgery, and in particular, to methods and apparatuses for sizing femoral components and placing reference marks to facilitate installation of femoral components.
2. Description of Related Art
Performance of a knee replacement surgery typically includes modification of one, or both, of the proximal end of the tibia and the distal end of the femur to have a shape that accommodates the tibial and femoral components, respectively, of the knee prosthesis. Modification typically involves some type of cutting procedure, e.g., with a bone saw, so as to expose, as exemplified by FIG. 15, planar surfaces 100, 101, 102, 103 and 104 on the femur F for attachment of the femoral component 110. An effective attachment of the femoral component 110 to the femur F is facilitated by cutting the femur F at appropriate depths and angles that match the dimensions and angles of the attachment (i.e., non-articulating) surfaces 111, 112, 113, 114 and 115 on the underside of the femoral component 110.
The femur, due to its complex geometry (e.g., lateral and medial condyles and intracondylar notch) can be particularly difficult to shape and therefore benefits greatly from accurate cuts. In addition, proper sizing of the components is important to ensure that the knee prosthesis has adequate stability and range of motion. To this end, various calipers and resection guides have been developed that measure the tibia and femur to determine appropriate sizes for the femoral and tibial components.
The ADVANCE® brand single reference point knee replacement system (Wright Medical, Inc., Arlington, TN) includes a femoral caliper that works off of a single reference point to size the femoral component. The knee replacement system also includes a guide for guiding subsequent cuts to the femur. During femur preparation, first, a starter hole is initiated in the femoral (medullary) canal between the condyles using a drill bit. A fluted intramedullary reamer is inserted into the femoral canal and is turned by hand by grasping and twisting a T-handle end attachment to reduce the occurrence of fat embolisms. An intramedullary alignment rod is then inserted into the reamed hole, preferably at a valgus angle of about 3° or 5°, and becomes the single point from which the remaining cuts are referenced.
A distal femoral resection guide is then assembled onto the intramedullary rod and is placed against the most prominent distal femoral condyle. The resection guide includes slots that are sized to support a bone saw (or other) blade as it cuts a section of the distal femur to form a flat surface in a “distal rough cut.” After the initial cut, the distal surface can then be gently planed until flat by a planer that is rotated about the intramedullary rod. The preferred amount of distal resection is between 9 mm and 13 mm.
After planing, the femoral sizing caliper is placed over the intramedullary rod and against the flat, resected distal femur. A pair of feet of the femoral sizing caliper are placed adjacent the posterior portion of the medial and lateral femoral condyles, while a feeler gauge is placed in contact with the anterior cortex of the femur. Movement of the feeler gauge and the feet into position registers a size on a sizing guide of the femoral sizing caliper.
Once the component size has been determined, an anterior-posterior resection guide corresponding to the previously measured size of the femoral component is placed over the intramedullary rod. The anterior-posterior resection guide is moved posteriorly until an anterior feeler gauge of the anterior-posterior resection guide contacts the anterior cortex of the femur. Additional anterior-posterior adjustments can be made using an anterior-posterior positioning screw which moves the resection guide in the anterior-posterior direction in small increments with respect to the feeler gauge. Proper positioning is confirmed by examining the anterior-posterior resection guide to ensure that about 2 mm to 4 mm of posterior condyle is visible under the resection guide.
A locking screw is tightened to secure the anterior-posterior resection guide once it has been positioned as desired and slots defined in the guide are used to complete various other cuts, such as the anterior and posterior chamfer cuts. Additional guides are attached in a similar manner to make other cuts. After all of the cuts have been made, the distal femur has a shape congruent to the attachment surface of the femoral component and the femoral component can be attached to the distal femur.
During this process, a sometimes difficult aspect of femoral component sizing is selecting a size for the femoral component when the femur, which has a natural biological variation in size and morphology from patient to patient, falls in between two sizes. The ADVANCE® brand single reference point knee replacement system generally recommends that a smaller size be selected in such a situation to avoid “over-stuffing” the knee in flexion, in other words, to avoid a knee that is too tight and resists full extension. Of course, selecting a smaller size can also result in a somewhat increased laxity of the joint at high knee flexion angles.
In addition to the ADVANCE® Single Reference Point brand knee replacement system, other systems are used for preparing the femur for attachment of a femoral prosthesis. Another single reference point system is described in U.S. Pat. No. 4,474,177 to Whiteside. Also, systems that do not use a single reference point can be employed. For example, there is also an ADVANCE® brand distal cut first knee replacement system that uses an intramedullary rod as a reference point to guide an initial distal cut. However, the remaining surgical steps, such as femoral component sizing and the anterior-posterior cuts, are referenced off of the resected distal femoral surface.
Despite the effectiveness of the above-listed knee replacement systems, additional improvements in systems and methods for preparing the distal femur for attachment of a femoral component are always desirable. Therefore, it would be advantageous to have a femoral caliper that can account for distal femurs that fall in between standard femoral component sizes. For instance, it would be advantageous if the femoral caliper were able to adjust the positioning of the femoral cuts (and hence the position of the femoral implant) to minimize tightness at low flexion angles and laxity at high flexion angles.